The detection of exhaust-gas relevant quantities of an internal combustion engine utilizing on board diagnostic equipment (OBD) is a stringent requirement of United States Federal Regulations. Combustion misfires, inter alia, must be detected utilizing on board means. Various methods exist for this purpose.
It is, for example, known to evaluate combustion misfires by evaluating the rough running of the crankshaft. For this purpose, the fluctuations of the crankshaft angular velocity are used during one or more camshaft rotations. Because of combustion misfires in one or several cylinders, the driving torque is not present and the angular velocity thereby becomes less because of the unchanged applied drive torque. Subsequent regular combustions accelerate the crankshaft again. The fluctuations of the angular velocity are that much more pronounced the lower the engine rpm and the higher the engine load. What is problematic about this method is that, for example, the misfiring of all cylinders at the same time is poorly detectable because this fault has the effect of an intended transition into the overrun mode of operation. In this case, the crankshaft angular velocity decreases uniformly. The method of detecting combustion misfires from fluctuations of the rpm is furthermore unsuitable at high rpms of the engine when these high rpms are associated with a low engine load.
Furthermore, methods are known to detect combustion misfires by evaluating the combustion chamber pressure. A pressure increase develops in the cylinders of the engine because of the combustion. This pressure can be detected by a sensor and be applied as a measure for the proper combustion.
What is problematic in this method is that an additional sensor is required for each cylinder. This sensor not only requires early consideration in the construction of the engine but especially causes additional costs.
Furthermore, the above-mentioned methods for detecting combustion misfires of an internal combustion engine via detecting the ion current signal are known. For example, a method of this kind is disclosed in U.S. Pat. No. 5,343,844 incorporated herein by reference. This patent also provides background information as to the ion current referred to above and hereinafter. It is advantageous in these methods that they require no additional sensor means and that the feature, which detects the combustion misfire, can be derived directly from the combustion operation. In this method, the ion current signal is either compared directly or after a time integration to a pregiven threshold value and, only when the threshold value is not exceeded, a fault signal is outputted. In these methods, it is problematic that smaller changes of the ion current signal can only be poorly detected and, for this reason, a fault can be signalled in extreme cases even though no combustion misfire is present.
The above is then especially the case when a relatively large offset signal is superposed upon an ion current signal caused by a combustion operation so that the actual ion current signal, which is caused by the combustion, can only be separated from the offset signal with great difficulty. Problems result here also with "flat" signal traces, that is, signal traces which are not adequately pronounced. In the integration of signals, which have a high offset, and because of the integration of the offset signal together with the actual signal to be detected, combustion misfires cannot be precisely detected under some circumstances. This is so because the integral of a very large offset signal can be greater than the pregiven threshold value and, for this reason, a proper combustion can be incorrectly assumed.
Because of the very large scattering of the ion current signal, it is not practical to subtract a certain offset quantity ab initio from the actual measuring signal.
Methods of this kind for detecting combustion misfires by detecting the ion current signal therefore impose high requirements on the evaluation of the signal. This is especially so because the ion current signals are very small compared to, for example, the ignition current and disturbances can also occur.